1. Field of the Invention
The present invention relates to a method of driving a solid-state image pickup device, and, more particularly, the present invention relates to a method of driving an all-pixel readout type solid-state image pickup device to provide increased dynamic range and improved image quality.
2. Description of the Prior Art
High-performance solid-state image pickup devices are being developed which have an increased number of horizontal pixels as well as a reduced physical size. However, for vertical resolution, it is only possible to obtain 350 TV lines with a field accumulation readout method which is compatible with the NTSC broadcasting system and 240 TV lines of resolution with a still image not using a mechanical shutter. This is because it is necessary to be compatible with the NTSC broadcasting format wherein two fields consisting of 262.5 scanning lines are interlaced in a 2:1 ratio to make one frame and also more importance has been placed on having the displayed image look smooth rather than increased vertical resolution. Conventional devices as shown in FIG. 7(A), have been designed so that signal charges Q1, Q2 of sensors (pixels) 71 of two adjacent horizontal lines are mixed by being read out into single packets 73 corresponding to two pixels in a vertical transfer register 72. In the next field signal charges Q2, Q1 are mixed by combining two different lines and a signal of 262.5 lines is outputted per one field, that is, with one exposure. The variation in charge amount inside a sensor part 71 is shown in FIG. 7(B).
However, in order to use a non-interlace system or still image fields, compared with the horizontal resolution, the vertical resolution is inadequate. In these fields, it is necessary to have vertical resolution as good as the horizontal resolution in one exposure. As a solution to this, there are solid-state image pickup devices of all-pixel readout type wherein packets 83 are provided in 1:1 correspondence with the pixels (sensor parts 81) of the vertical transfer register 82, as shown in FIG. 8(A). All pixels are read out independently without signal charges being mixed in the vertical transfer register 82. However, this all-pixel readout type solid-state image pickup device has a problem because after signal charges are photoelectrically converted and accumulated in the sensor parts 81, when the output becomes saturated due to an overload of sensor parts 81, the signal output becomes constant and it is not possible to obtain a signal output in proportion to the amount of incident light. The dynamic range with respect to light input is thus narrow. This is because the pixels become saturated when the light is very intense. This also occurs in the case of the solid-state image pickup device of field accumulation readout type shown in FIG. 7. The variation in charge amount in a sensor part 81 is shown in FIG. 8(B) in proportion to incident light.
In this regard, so-called high dynamic range readout type solid-state image pickup devices have been known wherein, all-pixel readout has been made possible by providing packets in the vertical transfer registers in 1:1 correspondence with the pixels. The dynamic range with respect to light input is effectively widened by, after reading out signal charges of the effective period of the vertical direction, providing a short exposure period immediately thereafter within the vertical blanking period. This subsequent signal charge is photoelectrically converted, and added to the signal output of the signal charge for the original longer exposure period. This is performed in a single processing line.
In this high dynamic range readout type solid-state image pickup device, as shown in FIG. 9(A), first, 1) a main signal charge Q1 of a pixel of an odd line (sensor part 91) is read out and immediately thereafter the vertical transfer register 92 is shifted by one pixel. Then after an auxiliary signal charge Q1' is accumulated in the pixel of the odd line again in a short exposure time(2) a main signal charge Q2 of a pixel of an even line is read out at the same time as the auxiliary signal charge Q1'. As a result, the main signal charge Q2 and the main signal charge Q1 are mixed and the auxiliary signal charge Q1' is read out into an empty packet 93. Then the vertical transfer register 92 is shifted by one pixel and then after an auxiliary signal charge Q2' is accumulated again in the pixel of the even line in a short exposure period (3) this auxiliary signal charge Q2' is read out. As a result of this process, the auxiliary signal charge Q2' is mixed with the auxiliary signal charge Q1'. The appearance of variation in the charge amount in a sensor part 91 is shown in FIGS. 9(b) and 9(c).
In this way, in a high dynamic range readout type solid-state image pickup device, signal charges are mixed in the vertical transfer register as in the case of the field accumulation readout type solid-state image pickup device, but the vertical transfer register 92 is used half-and-half by the main signal charges Q1, Q2 of the longer exposure time and the auxiliary signal charges Q1', Q2' of the shorter exposure time. A signal output based on the main signal charges (Q1+Q2) and a signal output based on the auxiliary signal charges (Q1'+Q2') are added in the signal processing line. As a result, as is clear from the incident light--signal output characteristic shown in FIG. 6, even after the accumulated signal charge has become saturated for the primary exposure, it is possible to obtain a signal output which is proportional to the actual incident light amount. Therefore it is possible to widen the dynamic range with respect to the light input. This is possible because although the primary exposure may have resulted in a maximum charge amount for the primary signal Q1 or Q2, the auxiliary signal charge provides information for a more accurate reading.
However, in a high dynamic range readout type solid-state image pickup device of the construction described above, because the main signal charge Q2 is read out after the accumulation period of the auxiliary signal charge Q1' has elapsed and accumulation of the auxiliary signal charge Q2' is carried out after that, a time difference of about half of the vertical blanking period arises between the readout time of the auxiliary signal charge Q1' and the readout time of the auxiliary signal charge Q2'. Because the exposure time of the auxiliary signal charges Q1', Q2' is extremely short compared to the exposure time of the main signal charges Q1, Q2, there is a time difference of about half of the vertical blanking period between the readout timings of the auxiliary signal charges Q1', Q2'. In particular, during filming of subjects moving at high speed, there is a problem because the auxiliary signal charges Q1', Q2' each contain information relating to different images. The resultant reproduced picture based on the auxiliary signal charges (Q1+Q2') becomes an unnatural one because the information for reproducing the image is comprised of image information from two different times. FIG. 9(b) and (c) illustrate how Q2 is generated only after the auxiliary charge Q1' is also generated, thus resulting in the above-mentioned timing problem.